(1) Field of the Invention
The present invention relates to a four wheel drive vehicle, and more particularly to a four wheel drive agricultural tractor comprising a pair of dirigible front wheels supported by front wheel support means, a propelling drive transmission disposed in a rear portion of the tractor, a pair of rear wheels receiving a drive from the propelling drive transmission, and a front wheel drive transmission receiving a drive divided out from the propelling drive transmission. The front wheel drive transmission is switchable between a first four wheel drive mode or a position for standard drive transmission in which an average peripheral speed of the pair of front wheels is substantially the same as an average peripheral speed of the pair of rear wheels, and a second four wheel drive mode or a position for acceleration in which the average peripheral speed of the pair of front wheels is greater than the average peripheral speed of the pair of rear wheels.
(2) Description of the Prior Art
In the agricultural tractor as described above, the front wheel drive transmission is switched to the accelerating position for accelerating the front wheels in response to a front wheel steering operation. Thus the tractor is pulled round by the front wheels when making a turn. This construction permits the tractor to make a smaller turn than the case of maintaining the front wheels at an average peripheral speed equal to an average peripheral speed of the rear wheels.
A known agricultural tractor of this type is disclosed, for example, in U.S. Pat. No. 4,574,910 in which a front wheel drive transmission is incorporated into a propelling drive transmission disposed in a rear portion of the tractor. The agricultural tractor often carries a rotary plow or other working implement connected to the rear end thereof. However, because of the weight of the working implement acting on the rear end of the tractor, a weight acting on the front wheels is smaller than when the working implement is not connected. It is necessary to provide a balancing weight in a front portion of the tractor in order that the front wheel acceleration be effective for the tractor to make a small turn. The balancing weight so provided must be very heavy since in the known construction the front wheel drive transmission imparts its weight to a greater degree to the rear wheels than to the front wheels.